Fish stocks are declining worldwide, yet conventional approaches to studying fish abundance and behavior rely heavily on imprecise local sonar and capture-trawling measurements. In particular, fish in continental-shelf environments have been monitored by line-transect techniques from slow moving research vessels; these techniques significantly under-sample fish populations in time and space, leaving an incomplete abundance and behavioral picture. Conventional fish-finding sonar (CFFS) operates in the 10-500 kHz range and measures the local depth distribution of fish by echo sounding within a narrow, downward-directed beam along the line transect of a slowly moving research vessel. Typically systems survey habitats at rates in the vicinity of 0.2 km2/hour, which is similar to the survey rates of capture-trawl vessels. Survey rates can increase by roughly an order of magnitude with conventional side-scan sonar, which exploits only local, linear, waterborne propagation paths. In contrast, isolated fish schools are often widely separated in space and difficult to detect by conventional methods. Small schools spanning hundreds of meters in diameter are known to undergo rapid variation in size and shape. Larger schools often extend over tens to hundreds of square kilometers and can also undergo drastic morphological changes, including fragmentation and clustering, in periods less than one hour.
Consequently, measurement of the size, spatial distribution, and temporal evolution of fish schools is generally not practical with conventional methods. The fish are too widely dispersed and, during the course of a measurement using CFFS, their spatial concentrations and distributions change dramatically.